Gradient composite coating, and preparation method and application thereof

Abstract

This invention belongs to the field of metal coating technology, specifically relating to a gradient composite coating, its preparation method, and its application. The composite coating of this invention comprises a multilayer Al3BC / Al composite material coating, wherein the Al3BC content in the Al3BC / Al composite material is 0 wt.% to 80 wt.%, and the Al3BC content increases or decreases sequentially from the inside to the outside of the multilayer coating, wherein the multilayer comprises at least three layers. The gradient Al3BC / Al composite coating of this invention balances high reflectivity, ablation resistance, lateral heat dissipation, longitudinal heat insulation, and low internal stress in laser protection scenarios, as well as high surface hardness, low interfacial stress, and high bonding strength in wear-resistant scenarios. Furthermore, the preparation method of the gradient Al3BC / Al composite coating is simple, with controllable parameters, and suitable for mass production. This invention also provides applications of this coating in the fields of laser protection coatings and wear-resistant coatings.

Description

Technical Field

[0001] This invention belongs to the field of metal coating technology, specifically relating to a gradient composite coating and its preparation method and application. Background Technology

[0002] High-energy laser weapons are rapidly developing towards practical application, miniaturization, and high power, posing a threat to the battlefield survivability of aircraft, missiles, armored vehicles, ships, and other weapon systems. Developing high-performance laser protective materials has become an urgent need in the defense sector. Laser protective coatings, due to their advantages of convenient application, strong adaptability, and high protective efficiency, have become the mainstream technical solution for resisting laser radiation and ensuring the reliable operation of equipment in high-laser environments.

[0003] In laser protection coating systems, pure aluminum coatings, with their excellent laser reflection properties, exhibit good reflectivity for typical military laser wavelengths such as 1064nm and 10.6μm, making them a research hotspot in the field of laser protection and leading to initial applications. However, pure aluminum suffers from a low melting point, poor high-temperature strength, and insufficient thermal stability, making it prone to melting, ablation, and oxidation failure under continuous high-energy laser irradiation. This makes it unsuitable for long-term protection against high-power-density lasers, severely limiting its engineering applications in high-end laser protection scenarios. To overcome the bottleneck of insufficient ablation resistance in pure aluminum coatings, researchers have attempted to introduce high-melting-point, high-stability ceramic reinforcing phases to construct aluminum-based composite material coatings.

[0004] Al3BC, as a novel ternary ceramic reinforcing phase, possesses characteristics such as low density, high hardness (20~25 GPa), high elastic modulus (326 GPa), and excellent high-temperature thermal stability. It undergoes a phase transition only above 1100℃ and exhibits a narrow bandgap and high dielectric constant, significantly enhancing the laser reflection and energy dissipation capabilities of aluminum substrates. Therefore, it is an ideal reinforcing particle for preparing high-performance aluminum-based laser protective coatings. Studies have shown that in-situ synthesized Al3BC particles can form a semi-coherent interface with the aluminum substrate, exhibiting high interfacial bonding strength and good wettability. This allows the ceramic phase to fully exert its reinforcing effect, effectively improving the high-temperature ablation resistance of aluminum-based coatings.

[0005] Chinese patent CN117089798A discloses a laser-protective Al3BC / Al metal matrix composite coating and its preparation method. The method achieves in-situ self-generation of Al3BC particles through plasma spraying or supersonic flame spraying, obtaining a homogeneous composite coating with an Al3BC mass fraction of 10-35 wt.%. This technology leverages the high-temperature stability and interfacial strengthening effect of Al3BC particles to improve the coating's resistance to laser ablation to some extent. However, the Al3BC particle content in the aforementioned composite coating is singular. While the introduction of Al3BC particles enhances the coating's ablation resistance, it also reduces the coating's laser reflectivity, resulting in limited laser protection and failing to meet the higher protection requirements brought about by the development of high-energy laser weapons.

[0006] In wear-resistant engineering applications, aluminum alloys are widely used in heavy-duty friction components in aerospace, rail transportation, and automobiles due to their advantages of being lightweight and having high specific strength. However, aluminum alloys themselves have low hardness and poor wear resistance, making them prone to adhesive wear and oxidative wear, resulting in short service life under harsh working conditions. Currently, a common method to improve wear resistance is to spray high-hardness ceramic coatings onto the surface of aluminum alloys. However, the significant differences in physical properties between the ceramic phase and the aluminum matrix lead to high internal stress and low bonding strength at the coating-matrix interface, making them prone to cracking, peeling, and other failures. The ultra-high hardness and modulus of Al3BC particles are key to constructing wear-resistant aluminum-based coatings. To obtain excellent wear resistance, the Al3BC mass fraction in the coating is usually high. However, a single high-content Al3BC / Al homogeneous coating also suffers from problems such as weak bonding with the aluminum matrix interface, poor stress matching, and insufficient mechanical compatibility, which restricts its stable application in heavy-duty wear-resistant components. Summary of the Invention

[0007] The technical problem this invention aims to solve is to overcome the shortcomings of existing homogeneous Al3BC / Al coatings, such as the difficulty in achieving a balance between reflectivity and ablation resistance in laser protection, and high interfacial stress and low bonding strength in wear-resistant applications. This invention provides a gradient composite coating, designed with a gradient distribution of Al3BC particle content along the coating thickness direction. This achieves a balance between high reflectivity, ablation resistance, lateral heat dissipation, longitudinal heat insulation, and low internal stress in laser protection scenarios, and high surface hardness, low interfacial stress, and high bonding strength in wear-resistant scenarios. Furthermore, this invention provides a method for preparing the gradient composite coating, which is simple, has controllable parameters, and is suitable for mass production. This invention also provides applications of this coating in the fields of laser protection coatings and wear-resistant coatings.

[0008] The composite coating of the present invention includes a coating of multilayer Al3BC / Al composite material, wherein the Al3BC content in the Al3BC / Al composite material is 0wt.% to 80wt.%, and the Al3BC content increases or decreases sequentially from the inside to the outside of the multilayer coating, wherein the multilayer includes at least three layers.

[0009] The gradient composite coating of this invention uses aluminum alloy as the matrix phase and Al3BC particles as the reinforcing phase. The Al3BC particle content is distributed in a continuous / stepwise gradient along the coating thickness direction, from high to low or from low to high, and the Al3BC mass fraction is controlled between 0 wt.% and 80 wt.%. The coating consists of multiple Al3BC / Al composite material layers, with no less than 3 layers, and optionally 3, 4, 5, or 6 layers.

[0010] The wear-resistant gradient composite coating of the present invention (with Al3BC content increasing sequentially from the inside to the outside): The Al3BC content in the coating increases stepwise from the inside out within the range of 5wt.% to 80wt.%, and the following typical formulation can be used: 5wt.%→15wt.%→25wt.%→35wt.%→50wt.%→80wt.%; 20wt.%→30wt.%→40wt.%→50wt.%→60wt.%; 10wt.%→30wt.%→40wt.%→65wt.%; 15wt.%→30wt.%→50wt.%.

[0011] In this structure, the Al3BC particle content of the composite coating decreases layer by layer from the surface inwards. The surface of the coating has the highest Al3BC particle content and the greatest hardness, which is beneficial to the wear resistance. The gradually decreasing Al3BC particle content can effectively reduce the internal stress between coatings with different Al3BC contents, improve the interfacial bonding strength, and prevent the coating from peeling off during the wear process.

[0012] The laser protection gradient Al3BC / Al composite coating of the present invention (with Al3BC content decreasing sequentially from the inside to the outside): The Al3BC content in the coating decreases in a stepwise manner from the inside out, ranging from 70 wt.% to 0 wt.%. A typical formulation can be adopted as follows: 50wt.%→30wt.%→15wt.%; 50wt.%→30wt.%→20wt.%→10wt.%; 65wt.%→45wt.%→35wt.%→25wt.%→10wt.%; 70wt.%→50wt.%→40wt.%→30wt.%→20wt.%→0wt.%.

[0013] In this structure, the composite coating features an Al3BC content that gradually increases from the surface inwards. Laser reflectivity decreases layer by layer, ablation resistance increases layer by layer, and thermal conductivity decreases layer by layer to promote lateral heat transfer and suppress longitudinal heat transfer. Simultaneously, the gradient content design effectively reduces internal stress within the coating and improves its bonding strength. This gradient structure design of the composite coating achieves a multi-effect synergistic laser protection mechanism of "high reflectivity + ablation resistance + lateral heat dissipation + longitudinal heat insulation + low internal stress." Compared to a single coating of the same thickness, this gradient coating exhibits superior laser protection performance.

[0014] The thickness of each layer of the composite coating is 50~300μm, and the total thickness of the composite coating is 300~1000μm.

[0015] The method for preparing the gradient composite coating includes the following steps: mixing Al3BC powder and aluminum-containing powder in different proportions to prepare Al3BC / Al composite powders with different Al3BC contents; drying and sieving the powders to obtain a composite powder particle size ≤100μm; coating the Al3BC / Al composite powders onto a substrate material with the Al3BC / Al composite powders having different Al3BC contents sequentially increasing or decreasing to obtain a gradient Al3BC / Al composite coating.

[0016] The base material is aluminum alloy, titanium alloy, magnesium alloy or steel.

[0017] The particle size of Al3BC powder is ≤10μm, preferably 0.05~5μm, and the particle size of aluminum-containing powder is ≤100μm, preferably ≤50μm. The aluminum-containing powder is aluminum powder or aluminum alloy powder.

[0018] The coating is supersonic flame spraying, with a spraying distance of 100~400mm, a spray gun lateral movement speed of 1000~3000mm / s, and a powder feeding rate of 1.9~10g / min.

[0019] Applications of the gradient composite coating: The composite coating with the Al3BC content increasing sequentially from the inside to the outside is applied to the field of wear-resistant coatings; the composite coating with the Al3BC content decreasing sequentially from the inside to the outside is applied to laser protective coatings.

[0020] The gradient composite coating according to the present invention comprises an aluminum alloy substrate and Al3BC particles. The aluminum alloy substrate has a high laser reflectivity, which is beneficial to reducing the accumulation of laser energy on the coating. The Al3BC particles have excellent high-temperature stability, which is beneficial to improving the ablation resistance of the coating. At the same time, its relatively narrow bandgap gives it a high laser reflectivity, resulting in a small reduction in the laser emissivity of the aluminum alloy. In addition, the introduction of Al3BC particles reduces the thermal conductivity of the coating, thus hindering the transmission of laser energy.

[0021] The gradient composite coating exhibits a gradient distribution of Al3BC particle content, ranging from high to low or low to high. Preferably, the gradient composite coating comprises 3 to 6 layers of Al3BC / Al composite coatings with varying Al3BC contents, extending from the surface inwards. The Al3BC particle content increases layer by layer, with the outermost layer containing 0 wt.% to 20 wt.%, the innermost layer containing 50 wt.% to 80 wt.%, and the intermediate transition layer containing 20 wt.% to 50 wt.%. This composite coating is applied in the field of laser protective coatings. The surface layer of the composite coating has an Al3BC particle content of no more than 20 wt.%, which ensures high laser reflectivity and effectively reduces laser energy deposition. From the surface inwards, the Al3BC particle content increases layer by layer, resulting in progressively improved ablation resistance. This ensures the coating is not damaged when laser ablation energy accumulates and causes a temperature increase. The innermost layer of the composite coating has an Al3BC particle content of no less than 50 wt.%, which ensures the inner layer... It exhibits excellent ablation resistance. Furthermore, with increasing Al3BC particle content, the thermal conductivity of the composite coating decreases layer by layer, preventing longitudinal heat propagation. The thermal conductivity at 25℃ for pure Al, 15% Al3BC / Al, 25% Al3BC / Al, and 50% Al3BC / Al composites are 238, 180, 164, and 36 W / m·K, respectively, while simultaneously promoting lateral heat dispersion. Finally, the gradient Al3BC content design reduces internal stress between coating layers, improves interfacial bonding strength, and prevents coating interface damage and detachment during laser ablation. In summary, this type of gradient composite coating achieves a multi-effect synergistic laser protection mechanism of "high reflectivity + ablation resistance + lateral heat dissipation + longitudinal heat insulation + low internal stress," demonstrating a laser protection effect far exceeding that of a single-content Al3BC / Al composite coating of the same thickness. This gradient coating is not a simple concentration superposition, but a synergistic innovation from gradient structure design to laser protection mechanism, ultimately achieving outstanding technical results.

[0022] As another preferred embodiment, the gradient composite coating comprises 3 to 6 layers of Al3BC / Al composite coatings with varying Al3BC contents, progressing from the surface inwards. The Al3BC particle content decreases layer by layer, with the outermost layer containing 50 wt.% to 80 wt.%, the innermost layer containing 5 wt.% to 20 wt.%, and the middle layers containing 20 wt.% to 50 wt.%. This type of composite coating can be applied to the field of wear-resistant coatings. The surface layer of the composite coating has the highest Al3BC particle content, and the high quality of Al3BC particles gives the surface coating extremely high hardness, ensuring the coating's excellent wear resistance. The gradual decrease in Al3BC particle content from the surface inwards effectively reduces internal stress caused by performance differences between coating layers, improves interfacial bonding strength, and avoids failure due to coating peeling.

[0023] Preferably, the particle size of Al3BC particles in the gradient composite coating is 0.05~5μm. Fine Al3BC particles have significant advantages in improving the strength and ablation resistance of the composite coating. The thickness of each composite coating layer is 50~300μm. By adjusting the thickness of each layer in the gradient composite coating, different thickness combinations can regulate its laser ablation resistance and wear resistance, thereby achieving further performance optimization.

[0024] According to the method for preparing the gradient composite coating of the present invention, the aluminum / aluminum alloy powder in the raw materials preferably has a particle size ≤100μm, more preferably ≤50μm; the Al3BC powder preferably has a particle size ≤10μm, more preferably ≤5μm. The fine particle size of the two raw materials helps to uniformly disperse Al3BC particles in the coating, effectively improving the strength of the coating.

[0025] According to the method for preparing gradient composite coatings of the present invention, the particle size of Al3BC / Al composite powder after sieving is preferably ≤100μm. This particle size range enables Al3BC / Al composite powder to have excellent flowability, which facilitates the transport of powder in pipelines and the stability of the powder during the subsequent spraying process. This is the basis for ensuring spraying efficiency and preparing high-quality coatings.

[0026] According to the method for preparing a gradient composite coating of the present invention, the supersonic flame spraying process has the unique advantages of high particle velocity and low particle temperature, which can significantly reduce the structural phase transformation of the coating and is an important spraying method for preparing high-performance coatings. The spraying distance is 100-400 mm, more preferably 150-300 mm, and even more preferably 150-250 mm; the spray gun traverse speed is 1000-3000 mm / s, more preferably 1000-2500 mm / s, and even more preferably 1000-2000 mm / s; the powder feed rate is 1.9-10 g / min, more preferably 5-8 g / min. The number of spray passes for each layer of the gradient composite coating is preferably 5-15, and even more preferably 10-15. A suitable spraying distance ensures that the powder is deposited on the substrate at a relatively gentle speed before reaching the substrate, effectively reducing the sputtering phenomenon caused by the high-speed impact of the powder, which is beneficial to improving the density of the coating and reducing porosity; a spray gun traverse speed of 1000~3000mm / s ensures uniform powder deposition during the spraying process, avoiding local accumulation or sparseness, and improving the coating quality; a powder feed rate of 1.9~10g / min ensures that the powder is heated evenly during the flame heating process, and the coating thickness can be controlled in combination with the high-speed traverse.

[0027] According to the method for preparing the gradient composite coating of the present invention, the gradient structure design results in a small difference in Al3BC content between layers, low internal stress in the coating, and each layer being an aluminum substrate coating, ensuring good compatibility and high interfacial bonding strength. By controlling the spraying distance (100-400 mm), the spray gun traverse speed (1000-3000 mm / s), and the powder feed rate (1.9-10 g / min) during the coating spraying process, the interfacial compatibility between the layers is further ensured, and the interfacial bonding strength is improved. This range of spraying process parameters effectively avoids powder sputtering, ensures a smooth powder deposition process, results in tight interlayer bonding, and guarantees the overall quality of the coating.

[0028] Compared with the prior art, the beneficial effects of the present invention are: (1) The gradient composite coating of the present invention, through the gradient distribution design of Al3BC particle content increasing or decreasing from the inside to the outside along the coating thickness direction, overcomes the technical defects of homogeneous Al3BC / Al coatings in the field of laser protection where reflectivity and ablation resistance are difficult to coordinate, and in the field of wear resistance where interface stress is high and bonding strength is low. When used for laser protection, the low Al3BC content on the surface layer ensures high laser reflectivity, while the high Al3BC content on the inner layer provides excellent ablation resistance. The gradient structure promotes lateral heat dissipation, inhibits longitudinal heat transfer, and reduces internal stress, achieving a multi-effect synergistic laser protection effect of "high reflectivity + ablation resistance + lateral heat dissipation + longitudinal heat insulation + low internal stress". When used for wear resistance, the high Al3BC content on the surface layer imparts ultra-high hardness, while the Al3BC content decreases layer by layer inward to effectively alleviate interface stress, improve bonding strength, and avoid coating wear and peeling failure.

[0029] (2) The preparation method of the present invention involves uniformly mixing Al3BC powder with aluminum-containing powder in different proportions and then coating it onto the substrate sequentially using a supersonic flame spraying process. The process parameters are controllable and the operation is simple. The optimized particle size of the powder ensures good flowability and deposition uniformity, resulting in a dense, low-porosity gradient coating with good interlayer bonding, which is suitable for mass production.

[0030] (3) The gradient composite coating of the present invention can be widely used in the fields of high-energy laser protection and heavy-duty wear resistance, significantly improving the equipment's resistance to laser ablation or wear life under extreme working conditions, and meeting the urgent need for high-performance coatings in the development of high-power laser weapons and harsh friction conditions. Attached Figure Description

[0031] Figure 1 This is a microstructure diagram of the gradient composite coating prepared in Example 1; Figure 2 This is the XRD pattern of the gradient composite coating prepared in Example 1; Figure 3 The microstructure diagrams of the 15wt.% Al3BC / Al composite coating of equal thickness prepared in Comparative Example 1 (left) and the 50wt.% Al3BC / Al composite coating of equal thickness (right) are shown. Figure 4 The gradient composite coating prepared in Example 1 (left image), the same thickness and single content 15 wt.% Al3BC / Al composite coating prepared in Comparative Example 1 (middle image), and the same thickness and single content 50 wt.% Al3BC / Al composite coating prepared in Comparative Example 2 (right image) are compared at 1500 W / cm². 2 Comparison of macroscopic ablation results after laser ablation; Figure 5The gradient composite coating prepared in Example 1 (left image), the composite coating of the same thickness with a single content of 15 wt.% Al3BC / Al prepared in Comparative Example 1 (middle image), and the composite coating of the same thickness with a single content of 50 wt.% Al3BC / Al prepared in Comparative Example 2 (right image) are compared at 1500 W / cm². 2 Comparison of back temperature during laser ablation process; Figure 6 This is a microstructure diagram of the gradient composite coating prepared in Example 5; Figure 7 The images show a comparison of the gradient composite coating (left image) prepared in Example 5, the substrate material (middle image), and a single 50wt.%Al3BC / Al composite coating (right image) after 10 minutes of friction and wear. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments. Example 1

[0033] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 15wt.%, 30wt.% and 50wt.% respectively, with the remainder being aluminum powder. Prepare three Al3BC / Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum powder is 10μm and the average particle size of Al3BC powder is 5μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤50μm.

[0034] (2) Supersonic flame spraying equipment was used for spraying. A 7A52 aluminum alloy substrate was selected as the spraying substrate. The spraying parameters for 15wt.% Al3BC / Al composite powder were set as follows: spraying distance 150mm, spray gun traverse speed 1500mm / s, and powder feed rate 2g / min. The spraying parameters for 30wt.% Al3BC / Al composite powder were set as follows: spraying distance 200mm, spray gun traverse speed 1000mm / s, and powder feed rate 2.4g / min. The spraying parameters for 50wt.% Al3BC / Al composite powder were set as follows: spraying distance 250mm, spray gun traverse speed 1500mm / s, and powder feed rate 2.8g / min. The powders were sprayed in descending order of Al3BC content, with each powder being sprayed 5 times for a total of 15 times. A gradient Al3BC / Al composite coating was prepared, with the innermost layer having an Al3BC mass fraction of 50 wt.%, the middle layer having an Al3BC mass fraction of 30 wt.%, and the outermost layer having an Al3BC mass fraction of 15 wt.%.

[0035] Figure 1The image shows the microstructure of the composite coating prepared in Example 1. As can be seen from the overall image a, the interfaces between the coatings are well bonded, the gradient coating is well bonded to the 7A52 aluminum alloy substrate, and the prepared coating is dense and defect-free. Figure 1 Figure b shows the microstructure of the outermost coating layer, with an Al3BC mass fraction of 15 wt.%. Figure 1 Figure c shows the microstructure of the intermediate coating, with an Al3BC mass fraction of 30 wt.%. Figure 1 The d-figure shows the microstructure of the innermost coating layer, with an Al3BC mass fraction of 50 wt.%. The Al3BC particle content gradually increases from the surface inwards in this gradient coating. The Al3BC particle size ranges from 50 to 600 nm. The coating thicknesses for 15 wt.%, 30 wt.%, and 50 wt.% Al3BC / Al coatings are 167, 194, and 139 μm, respectively, with a total coating thickness of 500 μm.

[0036] Figure 2 The XRD pattern of the composite coating prepared in Example 1 is shown, which confirms that the coating is composed of Al3BC particles and an aluminum matrix.

[0037] Figure 4 The images show the gradient composite coating prepared in Example 1 (left), the single-content 15wt.% Al3BC / Al coating prepared in Comparative Example 1 (middle), and the single-content 50wt.% Al3BC / Al coating prepared in Comparative Example 2 (right), at 1500 W / cm². 2 The comparison of macroscopic ablation results after 15s of ablation at power density clearly shows that the gradient coating of the present invention has a smaller ablation area and heat-affected zone compared with the single Al3BC / Al coatings of Comparative Examples 1 and 2, while the rest of the coating is unaffected. This confirms that the laser protection performance of the gradient composite coating is significantly better than that of the single Al3BC / Al composite coating under the same coating thickness.

[0038] Figure 5 The figure shows a comparison of the back temperature of the gradient composite coating prepared in Example 1 (left), the composite coating of similar thickness with a single content of 15 wt.% Al3BC / Al prepared in Comparative Example 1 (middle), and the composite coating of similar thickness with a single content of 50 wt.% Al3BC / Al prepared in Comparative Example 2 (right) as a function of laser ablation time. It can be seen from the figure that the temperature of the gradient coating is lower than that of the single coating at all time points during irradiation. In particular, at 15s of irradiation, the back temperature of the gradient coating is more than 100°C lower than that of the 15 wt.% Al3BC / Al single coating and more than 80°C lower than that of the 50 wt.% Al3BC / Al single coating. This fully demonstrates the significant advantage of gradient structure design in the laser protection application of Al3BC / Al composite coating at the same thickness.

[0039] It can be seen that the significant advantage of the gradient composite coating in laser protection of this invention is due to the gradient design of Al3BC content. The Al3BC content in this composite coating increases from the surface to the interior, with the highest laser reflectivity at the surface. As the first line of defense against laser damage, this effectively reduces laser energy deposition, with a surface reflectivity of 52.1%. Once the laser energy deposition exceeds a certain threshold, the surface coating is damaged. The progressively increasing Al3BC particles enhance its ablation resistance, ensuring good structural stability. Furthermore, as the Al3BC particle content increases, the thermal conductivity of the composite coating decreases layer by layer, effectively preventing longitudinal heat propagation while promoting lateral heat dispersion. The gradient Al3BC content design reduces internal stress between coating layers, improves interfacial bonding strength, and prevents coating interface damage and detachment during laser ablation. Therefore, the gradient structure design of this Al3BC / Al composite coating achieves a synergistic effect of high laser reflectivity, high laser ablation resistance, high lateral thermal conductivity, low longitudinal thermal conductivity, and low internal stress, exhibiting a laser protection effect far exceeding that of a single-content Al3BC / Al composite coating of the same thickness.

[0040] Comparative Example 1 This comparative example uses the same matrix material and Al3BC powder as Example 1 to prepare an Al3BC / Al composite powder with a content of 15 wt.% Al3BC. It undergoes the same treatment as Example 1, and is sprayed according to the spraying parameters for the 15 wt.% Al3BC / Al composite powder. The spray thickness is 504.2 μm, and its surface light reflectance is 51.4%. Its microstructure is shown in the figure. Figure 3 As shown in the left figure. At 1500W / cm 2 The macroscopic ablation result after laser ablation is shown in the figure below. Figure 4 As shown in the middle image. At 1500W / cm 2 Back temperature diagram during laser ablation process as shown in the figure Figure 5 As shown in the middle image.

[0041] Comparative Example 2 This comparative example uses the same matrix material and Al3BC powder as Example 1 to prepare an Al3BC / Al composite powder with a content of 50 wt.% Al3BC. It undergoes the same treatment as Example 1, and is sprayed according to the spraying parameters for a 50 wt.% Al3BC / Al composite powder, achieving a spray thickness of 508.7 μm and a surface light reflectance of 36.2%. Its microstructure is shown in the figure. Figure 3 As shown in the right figure. At 1500W / cm 2 The macroscopic ablation result after laser ablation is shown in the figure below. Figure 4 As shown in the right figure. At 1500W / cm 2 Back temperature diagram during laser ablation process as shown in the figure Figure 5 As shown in the right figure. Example 2

[0042] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 10wt.%, 20wt.%, 30wt.% and 50wt.% respectively, with the remainder being aluminum powder. Prepare four Al3BC / Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum powder is 10μm and the average particle size of Al3BC powder is 3μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤50μm.

[0043] (2) Spraying was carried out using a supersonic flame spraying device. A TC4 titanium alloy substrate was selected as the spraying substrate. The spraying parameters for 10wt.% Al3BC / Al composite powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 3.5g / min. The spraying parameters for 20wt.% Al3BC / Al composite powder were set as follows: spraying distance of 200mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 6.4g / min. The spraying parameters for 30wt.% Al3BC / Al composite powder were set as follows: spraying distance of 300mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 7.3g / min. The spraying parameters for 50wt.% Al3BC / Al composite powder were set as follows: spraying distance of 200mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 4.1g / min. The Al3BC content was sprayed sequentially from high to low, with each powder being sprayed in three passes for a total of 12 passes, to prepare a gradient Al3BC / Al composite coating with four layers. The Al3BC mass fractions from the surface to the inner layer were 10 wt.%, 20 wt.%, 30 wt.%, and 50 wt.%, respectively. The Al3BC / Al coating thicknesses were 107.4 μm, 110.7 μm, 117.2 μm, and 115.4 μm, respectively, with a total coating thickness of 450.7 μm and a surface light reflectance of 54.8%.

[0044] Comparative Example 3 This comparative example uses the same matrix material and Al3BC powder as Example 2 to prepare Al3BC / Al composite powder with a content of 10 wt.% Al3BC. It is treated in the same way as Example 1 and sprayed according to the spraying parameters of 10 wt.% Al3BC / Al composite powder. The spraying thickness is 451.3 μm, the surface light reflectance is 51.7%, and the back temperature is 375.8℃.

[0045] Comparative Example 4 This comparative example uses the same matrix material and Al3BC powder as Example 2 to prepare Al3BC / Al composite powder with a content of 50 wt.% Al3BC. It is treated in the same way as Example 1 and sprayed according to the spraying parameters of 50 wt.% Al3BC / Al composite powder. The spraying thickness is 450.2 μm, the surface light reflectance is 35.9%, and the back temperature is 372.4℃.

[0046] The coatings prepared in Example 2, Comparative Example 3, and Comparative Example 4 were subjected to a temperature of 1500 W / cm². 2 After 15 seconds of laser ablation, the back temperature of the four-layer gradient structure design in Example 2 was 255.1℃, which was 120.7℃ and 117.3℃ lower than that of the 10wt.%Al3BC / Al composite coating and the 50wt.%Al3BC / Al composite coating under the same total thickness conditions. Example 3

[0047] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 10wt.%, 25wt.%, 35wt.%, 45wt.% and 65wt.%, respectively, with the remainder being aluminum powder. Prepare five Al3BC / Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum powder is 10μm and the average particle size of Al3BC powder is 5μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤50μm.

[0048] (2) Spraying was performed using a supersonic flame spraying device. A ZK61M magnesium alloy substrate was selected as the spraying substrate. The spraying parameters for 10wt.% Al3BC / Al composite powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 8.5g / min. The spraying parameters for 25wt.% Al3BC / Al composite powder were set as follows: spraying distance of 200mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 2g / min. The spraying parameters for 25wt.% Al3BC / Al composite powder were set as follows: spraying distance of 300mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 2g / min. The spraying parameters for 35wt.% Al3BC / Al composite powder were set as follows: spraying distance 1500mm / s, spray gun traverse speed 1000mm / s, and powder feed rate 4.4g / min; and spraying distance 200mm, spray gun traverse speed 1500mm / s, and powder feed rate 9g / min. The Al3BC content was sequentially sprayed in descending order, with each powder being sprayed three times for a total of 15 times, to prepare a gradient Al3BC / Al composite coating with five layers. The Al3BC mass fractions from the surface to the inner layer were 10 wt.%, 25 wt.%, 35 wt.%, 45 wt.%, and 65 wt.%, respectively. The Al3BC / Al coating thicknesses were 132.4 μm, 113.2 μm, 107.5 μm, 117.4 μm, and 144.8 μm, respectively, with a total coating thickness of 615.3 μm and a surface light reflectance of 54.2%.

[0049] Comparative Example 5 This comparative example uses the same matrix material and Al3BC powder as Example 3 to prepare Al3BC / Al composite powder with a content of 10 wt.% Al3BC. It is treated in the same way as Example 3 and sprayed according to the spraying parameters of 10 wt.% Al3BC / Al composite powder. The spraying thickness is 617.8 μm, the surface light reflectance is 51.6%, and the back temperature is 369.8℃.

[0050] Comparative Example 6 This comparative example uses the same matrix material and Al3BC powder as Example 3 to prepare Al3BC / Al composite powder with a content of 65wt.% Al3BC. It is treated in the same way as Example 3 and sprayed according to the spraying parameters of 65wt.% Al3BC / Al composite powder. The spraying thickness is 612.8μm, the surface light reflectance is 34.8%, and the back temperature is 375.6℃.

[0051] The coatings prepared in Example 3, Comparative Example 5, and Comparative Example 6 were subjected to a temperature of 1500 W / cm². 2 After 15 seconds of laser ablation, the back temperature of the five-layer gradient structure design in Example 3 was 239.1℃, which was 130.7℃ and 136.5℃ lower than that of the 10wt.%Al3BC / Al composite coating and the 65wt.%Al3BC / Al composite coating under the same total thickness conditions, respectively. Example 4

[0052] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 0 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.% and 70 wt.%, respectively, with the remainder being aluminum powder. Prepare six Al3BC / Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum powder is 20 μm and the average particle size of Al3BC powder is 5 μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤100 μm.

[0053] (2) Supersonic flame spraying equipment was used for spraying. 45# steel substrate was selected as the spraying substrate. The spraying parameters for pure Al powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 2g / min. The spraying parameters for 20wt.% Al3BC / Al composite powder were set as follows: spraying distance of 300mm, spray gun traverse speed of 2000mm / s, and powder feed rate of 3.5g / min. The spraying parameters for 30wt.% Al3BC / Al composite powder were set as follows: spraying distance of 200mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 4.4g / min. The powder coating parameters are as follows: a spraying distance of 300 mm, a spray gun traverse speed of 1000 mm / s, and a powder feed rate of 5.2 g / min are used for the 40 wt.% Al3BC / Al composite powder; a spraying distance of 150 mm, a spray gun traverse speed of 1000 mm / s, and a powder feed rate of 6.4 g / min are used for the 50 wt.% Al3BC / Al composite powder; and a spraying distance of 350 mm, a spray gun traverse speed of 2000 mm / s, and a powder feed rate of 10 g / min are used for the 70 wt.% Al3BC / Al composite powder. The Al3BC content was sequentially sprayed in descending order, with each powder being sprayed twice, for a total of 12 coats, to prepare a gradient Al3BC / Al composite coating with six layers. The Al3BC mass fractions from the surface to the innermost layer were 0 wt.%, 20 wt.%, 30 wt.%, 40 wt.%, 50 wt.%, and 70 wt.%, respectively. The Al3BC / Al coating thicknesses were 74.3 μm, 60.4 μm, 79.6 μm, 82.3 μm, 86.5 μm, and 78.1 μm, respectively, with a total coating thickness of 461.2 μm and a surface light reflectance of 75.3%.

[0054] Comparative Example 7 This comparative example uses the same matrix material and Al3BC powder as Example 4 to prepare pure Al powder. It is treated in the same way as Example 4 and sprayed according to the spraying parameters of pure Al powder. The spraying thickness is 463.1 μm, the surface light reflectance is 72.6%, and the back temperature is 379.3℃.

[0055] Comparative Example 8 This comparative example uses the same matrix material and Al3BC powder as Example 4 to prepare Al3BC / Al composite powder with a content of 70 wt.% Al3BC. It is treated in the same way as Example 4 and sprayed according to the spraying parameters of 70 wt.% Al3BC / Al composite powder. The spraying thickness is 458.6 μm, the surface light reflectance is 33.7%, and the back temperature is 382.7℃.

[0056] The coatings prepared in Example 4, Comparative Example 7, and Comparative Example 8 were subjected to a temperature of 1500 W / cm². 2 After 15 seconds of laser ablation, the back temperature of the six-layer gradient structure design in Example 4 was 214.1℃, which was 165.2℃ and 168.6℃ lower than that of the 0wt.%Al3BC / Al composite coating and the 70wt.%Al3BC / Al composite coating under the same total thickness conditions, respectively, demonstrating superior laser protection performance. Example 5

[0057] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 50wt.%, 30wt.% and 15wt.% respectively, with the remainder being aluminum powder. Prepare three Al3BC / Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum powder is 20μm and the average particle size of Al3BC powder is 2μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤100μm.

[0058] (2) Supersonic flame spraying equipment was used for spraying. 6061 aluminum alloy substrate was selected as the spraying substrate. The spraying parameters for 15wt.% Al3BC / Al composite powder were set as follows: spraying distance of 150mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 8.5g / min. The spraying parameters for 30wt.% Al3BC / Al composite powder were set as follows: spraying distance of 200mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 6.4g / min. The spraying parameters for 50wt.% Al3BC / 6061Al composite powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 7.3g / min. The Al3BC content was sprayed sequentially in order of increasing value, with each powder being sprayed 5 times for a total of 15 times, to prepare a gradient Al3BC / 6061Al composite coating with three layers. The Al3BC mass fraction of the coating from the surface to the inside was 50 wt.%, 30 wt.%, and 15 wt.%, respectively.

[0059] Figure 6 The image shows the microstructure of the gradient composite coating prepared in Example 5. Good interfacial bonding between the coating layers can be observed. The Al3BC particle content gradually decreases from the surface layer to the inner layer. The Al3BC mass fractions from the surface to the inner layer are 50 wt.%, 30 wt.%, and 15 wt.%, respectively, with thicknesses of 160 μm, 154 μm, and 183 μm, respectively, for a total thickness of 497.4 μm. The gradient coating bonds well with the 6061 aluminum alloy substrate, and the prepared coating is dense and defect-free.

[0060] Figure 7 The figures show a comparison of the gradient composite coating prepared in Example 5 (left image a) and the uncoated 6061 substrate (middle image b) after 10 minutes of friction and wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min. Quantitative analysis using confocal microscopy revealed that after the same friction and wear test, the uncoated substrate material (magnified image e) had an average wear mark width of 1140 μm and an average wear mark depth of 61.1 μm; while the Al3BC / Al composite gradient coating (magnified image d) had an average wear mark width of 806 μm and an average wear mark depth of 44.2 μm. Clearly, the wear resistance of the gradient Al3BC / Al composite coating is significantly better than that of the 6061 substrate material. The surface layer of this type of composite coating has the highest Al3BC particle content. The high quality of Al3BC particles gives the surface coating extremely high hardness, ensuring the coating's excellent wear resistance. The Al3BC particle content decreases layer by layer from the surface inward, which can effectively reduce the internal stress caused by performance differences between coatings, improve the interfacial bonding strength, and avoid failure caused by coating peeling.

[0061] Comparative Example 9 This comparative example uses the same matrix material and Al3BC powder as Example 5 to prepare an Al3BC / Al composite powder with a content of 50 wt.% Al3BC. It undergoes the same treatment as Example 5, and is sprayed according to the spraying parameters for the 50 wt.% Al3BC / Al composite powder, with a spray thickness of 501.8 μm. The results after 10 minutes of friction and wear under conditions of 8 N load, 10 mm friction radius, and 200 r / min rotation speed are shown in the figure. Figure 7 As shown in (Figure c on the right).

[0062] After the coatings prepared in Example 5 and Comparative Example 9 were subjected to friction wear for 10 min under the conditions of 8 N load, 10 mm friction radius and 200 r / min rotation speed, the average wear mark width of the 50 wt.% Al3BC / Al composite coating was 953 μm, which is 147 μm higher than that of the gradient coating of the same thickness. Therefore, the gradient coating exhibits better wear resistance. Example 6

[0063] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Raw material powders were prepared with Al3BC powder accounting for 65wt.%, 40wt.%, 30wt.% and 10wt.%, respectively, and the remainder being 2024 aluminum alloy powder. Four Al3BC / 2024Al composite powders with different Al3BC contents were prepared by mechanical ball milling. The average particle size of the aluminum alloy powder was 20μm and the average particle size of the Al3BC powder was 5μm. The composite powders were sieved to ensure that the average particle size of the powders was ≤100μm.

[0064] (2) Supersonic flame spraying equipment was used for spraying. 2024 aluminum alloy substrate was selected as the spraying substrate. The spraying distance was set to 100 mm, the spray gun traverse speed was set to 3000 mm / s, and the powder feed rate was set to 7.3 g / min as the spraying parameters for 10 wt.% Al3BC / 2024Al composite powder. The spraying distance was set to 150 mm, the spray gun traverse speed was set to 3000 mm / s, and the powder feed rate was set to 7.3 g / min as the spraying parameters for 30 wt.% Al3BC / 2024Al composite powder. Spraying parameters for Al3BC / 2024Al composite powder: Spraying distance of 200mm, spray gun traverse speed of 3000mm / s, and powder feed rate of 8.5g / min were set for 40wt.% Al3BC / 2024Al composite powder; spraying distance of 250mm, spray gun traverse speed of 3000mm / s, and powder feed rate of 6.4g / min were set for 65wt.% Al3BC / 2024Al composite powder. The Al3BC content was sequentially sprayed in ascending order, with each powder applied in three passes for a total of 12 passes, resulting in a gradient Al3BC / 2024Al composite coating of four layers. The Al3BC mass fractions from the surface to the innermost layer were 65 wt.%, 40 wt.%, 30 wt.%, and 10 wt.%, with thicknesses of 106.3 μm, 116.8 μm, 121.6 μm, and 129.7 μm, respectively, for a total thickness of 474.4 μm. After 10 minutes of friction and wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the average wear mark width of the uncoated 2024 aluminum alloy substrate was 1028 μm, while the average wear mark width of the Al3BC / 2024Al composite gradient coating decreased to 740 μm, demonstrating excellent wear resistance.

[0065] Comparative Example 10 This comparative example uses the same matrix material and Al3BC powder as Example 6 to prepare an Al3BC / Al composite powder with a content of 65 wt.% Al3BC. The same treatment as in Example 6 was performed, and the powder was sprayed according to the spraying parameters for the 65 wt.% Al3BC / Al composite powder, resulting in a coating thickness of 471.2 μm. After 10 minutes of friction wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the 65 wt.% Al3BC / Al composite coating partially peeled off after the wear test. This was mainly due to the significant performance difference between the 65 wt.% Al3BC / Al composite coating and the matrix, resulting in poor interfacial bonding. This phenomenon did not occur in the gradient coating, indicating that the gradient structure design improved the interfacial bonding strength. Example 7

[0066] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Prepare raw material powders with Al3BC powder content of 60wt.%, 50wt.%, 40wt.%, 30wt.% and 20wt.%, respectively, and the remainder is A356 aluminum alloy powder. Prepare five Al3BC / A356Al composite powders with different Al3BC contents by mechanical ball milling. The average particle size of aluminum alloy powder is 10μm and the average particle size of Al3BC powder is 5μm. The composite powders are sieved to ensure that the average particle size of the powders is ≤50μm.

[0067] (2) High-velocity ultrasonic spraying equipment was used for spraying. A356 aluminum alloy substrate was selected as the spraying substrate. The spraying parameters for 20wt.% Al3BC / Al composite powder were set as follows: spraying distance 250mm, spray gun traverse speed 1000mm / s, and powder feed rate 2.4g / min; spraying distance 200mm, spray gun traverse speed 1000mm / s, and powder feed rate 5.2g / min; and spraying distance 300mm, spray gun traverse speed 150mm / s, and powder feed rate 1000mm / s. The spraying parameters for 40wt.% Al3BC / Al composite powder were set as follows: spraying distance 0mm / s, spray gun traverse speed 4.4g / min; spraying distance 350mm, spray gun traverse speed 1000mm / s, and powder feed rate 6.4g / min; and spraying distance 250mm, spray gun traverse speed 1500mm / s, and powder feed rate 8.5g / min. A gradient Al3BC / Al composite coating was prepared by spraying Al3BC powder in ascending order of Al3BC content, with three passes for each powder, for a total of 15 passes. The coating consisted of five layers, with Al3BC mass fractions of 60 wt.%, 50 wt.%, 40 wt.%, 30 wt.%, and 20 wt.% from the surface to the inner layers, and thicknesses of 118.6 μm, 102.8 μm, 97.6 μm, 124.7 μm, and 106.2 μm, respectively, for a total thickness of 449.9 μm. After 10 minutes of friction and wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the average wear mark width of the uncoated A356 aluminum alloy substrate was 950 μm, while the average wear mark width of the Al3BC / A356 composite gradient coating decreased to 670 μm, demonstrating excellent wear resistance.

[0068] Comparative Example 11 This comparative example uses the same matrix material and Al3BC powder as Example 7 to prepare an Al3BC / Al composite powder with a content of 60 wt.% Al3BC. The same treatment as in Example 7 was performed, and the powder was sprayed according to the spraying parameters for the 60 wt.% Al3BC / Al composite powder, resulting in a coating thickness of 554.2 μm. After 10 minutes of friction wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the average wear mark width of the 60 wt.% Al3BC / Al composite coating was 956 μm, which is 286 μm wider than that of a gradient coating of the same thickness. Therefore, the gradient coating exhibits superior wear resistance. Example 8

[0069] The method for preparing the gradient composite coating of the present invention includes the following steps: (1) Raw material powders were prepared with Al3BC powders of 80wt.%, 50wt.%, 35wt.%, 25wt.%, 15wt.% and 5wt.%, respectively, and the remainder was 7050 aluminum alloy powder. Six Al3BC / 7050Al composite powders with different Al3BC contents were prepared by mechanical ball milling. The average particle size of the aluminum alloy powder was 10μm and the average particle size of the Al3BC powder was 3μm. The composite powders were sieved to ensure that the average particle size of the powders was ≤50μm.

[0070] (2) Supersonic flame spraying equipment was selected for spraying. A 7050 aluminum alloy substrate was selected as the spraying substrate. The spraying parameters for 5wt.% Al3BC / 7050Al composite powder were set as follows: spraying distance of 150mm, spray gun traverse speed of 1500mm / s, and powder feed rate of 1.9g / min. The spraying parameters for 15wt.% Al3BC / 7050Al composite powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 2.5g / min. The spraying parameters for 25wt.% Al3BC / 7050Al composite powder were set as follows: spraying distance of 250mm, spray gun traverse speed of 1000mm / s, and powder feed rate of 3.6g / min. Spraying parameters for Al3BC / 7050Al composite powder: Spraying distance 250mm, spray gun traverse speed 1500mm / s, powder feed rate 4.8g / min for 35wt.% Al3BC / 7050Al composite powder; Spraying distance 300mm, spray gun traverse speed 1000mm / s, powder feed rate 5.5g / min for 50wt.% Al3BC / 7050Al composite powder; Spraying distance 150mm, spray gun traverse speed 2000mm / s, powder feed rate 2.8g / min for 80wt.% Al3BC / 7050Al composite powder. A gradient Al3BC / 7050Al composite coating was prepared by sequentially spraying Al3BC powders in ascending order of content, with two passes for each powder, for a total of 12 passes. The coating consisted of six layers, with Al3BC mass fractions of 80 wt.%, 50 wt.%, 35 wt.%, 25 wt.%, 15 wt.%, and 5 wt.% from the surface to the inner layers, and thicknesses of 113.5 μm, 108.9 μm, 121.2 μm, 126.8 μm, 105.3 μm, and 114.3 μm, respectively, for a total thickness of 690 μm. After 10 minutes of friction and wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the average wear mark width on the uncoated 7050 aluminum alloy substrate was 984 μm, while the average wear mark width of the Al3BC / 7050Al composite gradient coating decreased to 590 μm, confirming the excellent wear resistance of the gradient coating.

[0071] Comparative Example 12 This comparative example uses the same matrix material and Al3BC powder as Example 8 to prepare an Al3BC / Al composite powder with an 80 wt.% Al3BC content. It undergoes the same treatment as Example 8, and is sprayed according to the spraying parameters for the 80 wt.% Al3BC / Al composite powder, with a coating thickness of 686.3 μm. After 10 minutes of friction wear under a load of 8 N, a friction radius of 10 mm, and a rotation speed of 200 r / min, the 80 wt.% Al3BC / Al composite coating exhibits peeling failure, confirming that the gradient content design results in higher interfacial bonding strength and fully utilizes its wear resistance.

[0072] The coatings prepared in the above embodiments and comparative examples were tested according to the standard. The test results are shown in Table 1. Back temperature refers to the temperature at the center point of the back of the substrate, which was measured using a K-type thermocouple.

[0073] Table 1. Performance test results of the examples and comparative examples.

Claims

1. A gradient composite coating, characterized in that: The composite coating comprises a multilayer Al3BC / Al composite material coating, wherein the Al3BC content in the Al3BC / Al composite material is 0wt.% to 80wt.%, and the Al3BC content increases or decreases sequentially from the inside to the outside of the multilayer coating, wherein the multilayer comprises at least three layers.

2. The gradient composite coating according to claim 1, characterized in that: The Al3BC content increases sequentially from the inside to the outside of the coating, ranging from 5wt.% to 80wt.%.

3. The gradient composite coating according to claim 2, characterized in that: The Al3BC content increases sequentially within the range of 5wt.% to 80wt.%, including 5wt.% → 15wt.% → 25wt.% → 35wt.% → 50wt.% → 80wt.%. 20wt.%→30wt.%→40wt.%→50wt.%→60wt.%, 10wt.%→30wt.%→40wt.%→65wt.% or 15wt.%→30wt.%→50wt.%.

4. The gradient composite coating according to claim 1, characterized in that: The Al3BC content decreases sequentially from the inside to the outside of the coating, ranging from 70wt.% to 0wt.%.

5. The gradient composite coating according to claim 4, characterized in that: The Al3BC content decreased sequentially within the range of 70 wt.% to 0 wt.%, including 50 wt.% → 30 wt.% → 15 wt.%, and 50 wt.% → 30 wt.% → 20 wt.% → 10 wt.%. 65wt.%→45wt.%→35wt.%→25wt.%→10wt.% or 70wt.%→50wt.%→40wt.%→30wt.%→20wt.%→0wt.%.

6. The gradient composite coating according to claim 1, characterized in that: The thickness of each layer of the composite coating is 50~300μm, and the total thickness of the composite coating is 300~1000μm.

7. A method for preparing a gradient composite coating according to any one of claims 1-6, characterized in that, The process includes the following steps: mixing Al3BC powder and aluminum-containing powder in different proportions to prepare Al3BC / Al composite powders with different Al3BC contents; drying and sieving the powders; and coating the Al3BC / Al composite powders onto a substrate material in sequence, with the Al3BC / Al composite powder contents increasing or decreasing sequentially as set, to obtain a gradient composite coating.

8. The method for preparing the gradient composite coating according to claim 7, characterized in that: The particle size of Al3BC powder is ≤10μm, and the particle size of aluminum-containing powder is ≤100μm.

9. The method for preparing the gradient composite coating according to claim 7, characterized in that: The coating is a supersonic flame spray, with a spraying distance of 100~400mm, a spray gun lateral movement speed of 1000~3000mm / s, and a powder feeding rate of 1.9~10g / min.

10. An application of the gradient composite coating according to any one of claims 1-6, characterized in that: The composite coating with the Al3BC content increasing sequentially from the inside out is used in the field of wear-resistant coatings; the composite coating with the Al3BC content decreasing sequentially from the inside out is used in laser protective coatings.

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